A lithium secondary battery is expected to be applied not only to small IT equipment, such as a mobile phone and a notebook PC, but also as a medium- and large-size battery for an electric car and an electric power storage system. Particularly, there is a demand for development of an anode material with high safety and high energy density needed for a medium- and large-size battery for an electric car and an electric power storage system. Generally, for a lithium secondary battery, electrode materials that are based on LiCoO2 and have safety and excellent capacity, that is, LiMn2O4 (LMO) and high-capacity LiMn1/3Co1/3Ni1/3O2 (NMC), have been studied. However, as such materials have a low basic capacity or are not yet satisfactory in safety, safe materials with high energy density for commercialization of medium- and large-size batteries are sought.
In particular, a driving distance on a single charge, which is very important for an electric car, is associated with the energy density of an anode material for a secondary battery, research and development for achieving high performance of the anode materials is essential. Traditional LMO, NMC or olivine-based anode materials have an energy density of about 120 to 150 mAh/g, which is insufficient to dramatically increase the driving distance of an electric car.
An Li2MnO3-based anode composite material has a high basic theoretical capacity of about 460 mAh/g, a high actual initial capacity of 200 mAh/g or higher, and a relatively high average discharge voltage of about 3.5 V. Therefore, the Li2MnO3-based anode composite material is known as one of next-generation anode material candidates capable of achieving high capacity and high energy density and, thus, technologies of synthesizing, with high efficiency, such anode materials that are highly likely to achieve high performance are examined.
A medium- and large-size lithium secondary battery for an electric car and an electric power storage system is first required to have safety and high energy density. Thus, in the prior art, to secure safety of a medium- and large-size lithium secondary battery, research and development of a process of preparing a mixture of a lithium manganese oxide (LMO) spinel material and an NMC material with a relatively high capacity at an appropriate composition or a process of preparing an olivine-type lithium-ion iron phosphate (LiFePO4), which exhibits a relatively low discharge voltage of about 3.0 V but has excellent safety and high capacity, is being conducted. However, due to low basic capacity, batteries of these traditional LMO, NMC and olivine-type LiFePO4 materials have limitations in improving the driving distance on a single charge of an electric car.
Traditional anode materials have a basic energy density of about 120 to 150 mAh/g, which is insufficient, and thus have limitations in commercialization of applications thereof that need a high energy density, such as electric cars. In particular, since iron phosphate materials, which have attracted attention in recent years, have a low voltage and clear limitation in capacity increase (3 V and 150 mAh/g), it is urgent to develop an anode material with a superior energy density. For reference, batteries of conventional electrode materials mostly operate in a charge voltage range of 2.0 to 4.2 V.
Traditional anode materials have problems in terms of safety, cost and high energy density. Specifically, nickel-based materials, such as LiNiO2 (LNO), having excellent capacity, are not practical in safety; manganese LMO materials are not practical in capacity and durability; NMC materials have problems in safety and cost; and iron phosphate materials have problems in energy density and cost.
In particular, manganese materials are excellent in safety but have a low capacity and are not sufficiently identified in terms of durability, and thus research and development of manganese materials are vigorously being conducted. Also, although studies on a nanotechnology for iron phosphate materials to obtain high-capacity electrode performance are being carried out, the nanotechnology for the materials involves an additional cost increase. Thus, batteries of these traditional materials are charged and discharged in a range of about 2.0 to 4.2 V to secure safety in view of properties of the materials and thus basically have limitations in discharge capacity.